A recent report from the National Center for Health Statistics showed that for the first time in over 70 years there was a decline in the actual number of cancer deaths in the United States. Although it is uncertain whether the decline in the total number of cancer deaths will continue, the decline marks an important landmark in the battle against cancer. However, estimates included in the 55th edition of Cancer Facts & Figures project that in 2006, approximately 1.4 million Americans will be diagnosed with cancer and 565,000 will die of the disease. In order to continue to decrease the number of cancer-related deaths, continued development of anti-cancer therapeutics is essential.
Current anti-cancer therapies are generally divided into four major categories: surgery, radiation, chemotherapy, and biologic therapies. Each of these therapies provides certain advantages as well as major drawbacks. For example, although the clinical use of chemotherapeutic agents against cancerous tumors is successful in many cases, they have limited efficacy in many other cases and can cause severe side-effects that limit therapeutic usefulness. These limited efficacies as well as the severe side effects are often the result of a lack of selectivity. Recently, laser-induced thermal and accompanying effects around nanoparticles (NPs) functionalized with specific—bio-ligands have been explored for selective photothermal (PT)-based cancer therapy, suggesting high potentials of PT-based nanotherapeutics for in vivo cancer treatment.
Recent advances in nanotechnology have provided a variety of nanostructured materials with unique and highly controlled properties including exceptionally high strength, the ability to carry conjugates to targets and unique optical properties. By controlling structure at the nanoscale, the properties of nanostructures such as, for example, semiconductor nanocrystals and metal nanoshells can be controlled in a very predictable manner. In addition, the similarity in size of many NPs to biomolecules enables them to be used for applications such as intracellular tagging and makes them ideal candidates for bioconjugate applications such as antibody (Ab) targeting of contrast agents. Thus, these materials can bring new and unique capabilities to a variety of biomedical applications ranging from diagnosis of diseases to novel therapies.
Many of the biomedical and biological applications of nanotechnology involve bioconjugates. The idea of interfacing biological and non-biological systems at the nanoscale level has been investigated for many years. The broad field of bioconjugate chemistry combines the functionalities of biomolecules and abiologically derived molecular species for applications such as markers for research in cellular and molecular biology, biosensing, and biomedical and biological imaging. Major challenges of bioconjugate chemistry include stable and ‘controllable’ integration and interfacing of the bio- and abio-materials. The stable and ‘controllable’ interfaces enable us to develop reliable and predictable systems. For example, multiple genes are known to be involved in many diseases, such as, for example, breast and ovarian cancers. However, only a fraction of such genes have been identified and correlated with a particular disease, even after the completion of sequencing of the human genome. In addition, there are more and more observations of alternative splicing, post-translational modifications, and proteolysis of proteins. These discoveries have generated a renewed demand for multiplex bindings of different biocomponents, such as, for example, DNA or Ab, to effectively detect cancerous cells.
Nanostructured materials possess optical properties far superior to the molecular species they replace, such as, for example, higher quantum efficiencies, greater scattering or absorbance cross sections, optical activity over more biocompatible wavelength regimes, and substantially greater chemical stability or stability against photobleaching. Additionally, some nanostructures provide optical properties that are highly dependent on particle size or dimension. The ability to systematically vary the optical properties via structure modification not only improves traditional applications, but also may lead to applications well beyond the scope of conventional molecular bioconjugates.
Quantum dots are nano-sized crystals composed of transition metals such as cadmium, selenium, and technetium, and are highly light absorbing, luminescent NPs whose absorbance onset and emission maximum shift to higher energy with decreasing particle size due to quantum confinement effects. These nanocrystals are typically in the size range of 2-8 nm in diameter. Unlike molecular fluorophores, which typically have very narrow excitation spectra, semiconductor nanocrystals absorb light over a very broad spectral range. This makes it possible to optically excite a broad spectrum of quantum dot “colors” using a single excitation laser wavelength, which may enable one to simultaneously probe several markers in biosensing and assay applications. However, these advantages cannot be realized without first considering the biocompatibility of the materials used. Because the dots are composed of heavy metals, which can be toxic, they have not yet been approved for use in humans. The risk of the heavy metals versus the benefit of obtaining vital information must be weighed.
In many cases, modifications to nanostructures can make them better suited for integration with biological systems; for example, modification of their surface layer may enhance aqueous solubility, biocompatibility, or biorecognition. Nanostructures can also be embedded within other biocompatible materials to provide nanocomposites with unique properties. Surface functionalization with molecular species such as mercaptoacetic acid or the growth of a thin silica layer on a NP's surface may facilitate aqueous solubility. Both the silica layer and the covalent attachment of proteins to the mercaptoacetic acid coating permit the NPs to be at least relatively biocompatible. Quantum dots have also been modified with dihydrolipoic acid to facilitate conjugation of avidin and subsequent binding of biotinylated targeting molecules. Quantum dots can also be embedded within polymer nano- or microparticles to improve biocompatibility while maintaining their unique fluorescence. Specific binding of quantum dots to cell surfaces, cellular uptake, and nuclear localization have all been demonstrated following conjugation of semiconductor nanocrystals to appropriate targeting proteins, such as transferrin or antibodies (Abs).
Embedment of components, such as, for example, bioconjugated NPs, within a carrier has been examined for the potential to avoid physiological barriers. The carrier may be designed to confer solubility and the ability to circulate in the system to avoid accumulation in the liver or spleen. In addition, the carrier may be designed to effectively release the internal components when reaching the target site. Polymer carriers have a greater potential to meet the above requirements over other delivery methods such as liposomes. Because liposomes, spherical vesicles made of ionic lipids, are particles, they are taken up by macrophages. High levels can be found in the liver and spleen, even when the liposomes are coated with polymers. Coated liposomes have other side effects, such as extravasation, in which the liposome moves from the blood vessel into tissue where it may not be wanted. Uncharged hydrophilic polymers, such as polyethylene glycol (PEG) and N-(2-hydroxypropyl) methacrylamide might enable avoidance of accumulation in the liver and spleen. When these polymers are hydrated, they can circulate in the blood for periods of about 1 hour, or 3 hours, or 6 hours, or 12 hours, or 24 hours, or 48 hours or longer. Another advantage of polymers is that the linkage can be designed to control where and when the drug is released. In addition, polymer vesicles have the advantages of superior stability and toughness and offer numerous possibilities for tailoring physical, chemical, and biological properties by variation of block lengths, chemical structure, and conjugation with biomolecules. While liposomes are basically empty vesicles that may be filled with a compound such as a drug, polymers may have a lower drug-carrying capacity. Exemplary use of these vesicles in controlled transmembrane transport and DNA-encapsulation, and controlled release of plasmids for gene transfection have been demonstrated.
Gold colloid has been used in biological and biomedical applications since 1971 when the immunogold staining procedure was invented. The labeling of targeting molecules, such as Abs, with gold NPs has revolutionized the visualization of cellular components. The optical and electron beam contrast properties of gold colloid have provided excellent detection capabilities for applications including immunoblotting, flow cytometry, and hybridization assays. Furthermore, conjugation protocols to attach proteins to gold NPs are robust and simple, and gold NPs have been shown to have excellent biocompatibility.
Gold nanoshells, a new type of gold NPs, are concentric sphere NPs consisting of a dielectric core NP, such as, for example, gold sulfide or silica, surrounded by a thin gold shell. The plasmon-derived optical resonance of gold can be dramatically shifted in wavelength from the visible region into the mid-infrared by varying the relative dimensions of the core and shell layers. Nanoshells may be designed to either strongly absorb (particles less than 75 nm) or scatter the incident light, depending upon their sizes. The gold shell layer is formed using the same chemical methods that are employed to form gold colloid. Thus, the surface properties of gold nanoshells are virtually identical to gold colloid, providing the same ease of bioconjugation and excellent biocompatibility. Intravenous injection of 130 nm gold nanoshells with a maximum absorption near 800 nm followed by continuous-wave laser irradiation has been used to successfully destroy a localized tumor in an animal model.
Gold carbon nanotube (gCNT) rods, are a new type of gold NPs that are concentric rods consisting of a carbon nanotube (CNT) core (either single-walled or multi-walled), surrounded by a thin gold layer. The gCNT is developed by uniquely combining the optical property of CNT and the biocompatibility and bioconjugation potential of gold. Carbon nanotubes are known to have strong absorbance from about 700- to 1,100-nm NIR light, in which biological systems are transparent. Gold is proven to be biocompatible, and is one of a few metals approved by the FDA for human uses. However, gold is not responsive to NIR, limiting its medical application for disease diagnosis and therapy. Therefore, the advantages of both CNT and gold are realized by interfacing gold with CNT. In fact, their interfacing through a chemical reduction process results in an NIR responsive concentric gold nanotube that comprises a CNT core and a layer of gold surrounding the CNT. The similar methods used to form gold colloids are used to form gCNT; thus, gCNT provides excellent biocompatibility and the same simple and robust bioconjugation as gold colloid. Furthermore, GNTs are NIR responsive and the surface plasmon resonance of GNT shows the transverse plasmon absorption at visible region of 520-530 nm as in spherical Au NPs as well as their longitudinal resonance peak in the NIR region near 850 nm as in GNRs. The combination of the CNTs' inherent NIR absorption and the plasmon effects in Au layer around a long CNT tube with very small diameter enable GNTs' synergistic NIR absorption enhancement. Therefore, the NIR responsiveness and biocompatibility of the gCNT allow non-invasive diagnosis and therapy of diseases, such as, for example, cancer.
Furthermore, the ability of CNTs to be spontaneously internalized by cells has recently excited numerous studies on transporting peptides, DNA, and RNA inside cells, both eukaryotic and prokaryotic, for tissue specific gene/drug delivery. The gCNT's smaller diameter, rod-like shape with hollow core, and less rigidity (close to CNT's mechanical properties) could provide better penetration to cells as compared to spherical gold nanoshells, thereby overcoming a limitation in the use of this class of nanostructures in the photothermal therapy of cancer. In addition, gCNTs could provide better targeting to small cell-surface biomolecules (5-10 nm), better clustering capability, and possibility to carry therapeutic payloads in their hollow cores.